PU weighting

Adding weighting of neutral PF candidates to the PF sequence. This is more powerful treatment of neutral PU isolation than simple delta beta corrections (see https://indico.cern.ch/event/322762/ and slides by Yuta). The metric used is pt/DeltaR2. On top of original design by @bachtis added also interface to reco::Candidates and so the reweighting module should be able to run also on miniAOD.